The present invention relates to automatic gain control (referred suitably to as “AGC” hereinafter) which is able to stabilize an input signal whose level is varied in a mobile communication device, etc. via gain control using a closed loop and then output it and, more particularly, an automatic gain control method and a device for embodying the same, which is able to execute the automatic gain control based on a transmission signal level (received signal strength indicator (RSSI) signal, etc.) detected from the input signal when a signal-to-noise (S/N) ratio of the input signal is degraded, and a radio communication device with such automatic gain control function.
In the prior art, in various signal processing devices, e.g., in the radio communication device, the automatic gain control (AGC) which stabilizes variation in a signal level of the received signal caused due to variation in a received electric field strength on a radio wave propagation path such as a radio line, etc. to reduce demodulation error, etc. is applied. As such automatic gain control, there are known the open loop control device in which a gain of-a variable amplifier, etc. at the succeeding stage is controlled by an automatic gain control signal generated by detecting variation in the level of the input signal, and the close loop control device in which the gain of the variable amplifier, etc. at the preceding stage is controlled by the automatic gain control signal generated by detecting variation in the level of the input signal.
Under such automatic gain control, if the S/N of the input signal is degraded and thus a level of the input signal is reduced below a level of the noise signal, it becomes impossible to detect the variation in the level of the input signal. That is, it becomes impossible to generate an automatic gain control signal based on the input signal, so that the gain control of the variable amplifier at the preceding stage in the close loop control device cannot be carried out in answer to the variation of the input signal. In other words, if the device is employed under the noisy surroundings like the case the mobile telephone, the radio paging device (pager), or the like, for example, is employed with the movement under the surroundings with the extremely high noise level (surroundings in which there are much ignition noises of the vehicle, much radiation noises of electronic devices, etc.), or if the input signal level (received electric field strength) is low, the proper automatic gain control for the input signal cannot be achieved. In this case, there are such problems that good receiving characteristics cannot be assured and thus the received signal cannot be derived at a desired level.
As the digital AGC device, an example set forth in Patent Application Publication (KOKAI) Hei 9-83276 entitled “AGC Amplifier Control Circuit” is well known. In this example set forth in Patent Application Publication (KOKAI) Hei 9-83276, a gain obtained when the previous introduction of the input signal has been completed can be maintained at the time of starting the introduction of the input signal, so that this maintained gain is set in an AGC amplifier as an initial value and also the introduction error can be controlled by using a digital AGC loop. Accordingly, it is possible to delete the primary introduction of the input signal in an analogue format and thus to achieve high speed amplitude introduction control.
In the above-mentioned prior art, there is the problem that, when an S/N of the input signal is degraded and thus a level of the input signal is reduced below a level of the noise signal, reduction in a control voltage of a gain variable amplifier is caused and therefore proper automatic gain control cannot be achieved. That is, there is such a drawback that good receiving characteristics of the device cannot be achieved.